Cambodia’s strongman prime minister Hun Sen has implemented a brutal crackdown on dissent in a move aimed at quashing one of the most serious challenges to his three-decade long rule.

Opposition leaders, fearing more bloodshed after the killing of four people during a garment worker’s demonstration on Friday, called off a major protest rally planned for Sunday.

Cambodian authorities have banned protests and street marches in Phnom Penh after unidentified men in plainclothes wielding steel bars, metal pipes, batons, sticks and axes forcibly cleared hundreds of demonstrators from their rally base in the capital on Saturday.

Monks and women were among those chased and beaten, witnesses said.

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Until Friday’s shootings Cambodian authorities had shown restraint in handling protests while Mr Hun Sen remained firmly in control of the police and military.

A former cadre of the murderous Khmer Rouge who defected to Vietnam before becoming Asia’s youngest leader 28 years ago, Mr Hun Sen has shown in the past he is capable of instigating violence, as in a 1997 putsch that overthrew his then senior coalition partner Prince Norodom Ranariddh.

The country’s Defence Ministry has taken the unusual step of issuing a statement affirming the military’s loyalty to the government, the king and constitution.

The ministry said the armed forces blamed “opportunists” and politicians who had insulted the government and incited people to oppose Mr Hun Sen’s leadership by provoking instability.

Government officials denied rumours sweeping social media that key opposition and labour figures were to be arrested, including opposition leader Sam Rainsy who returned to the country days before last July’s disputed national elections.

But Mr Rainsy and his deputy Kem Sokha have been summoned to attend Phnom Penh municipal hearings on January 14 to answer allegations they may have incited crimes and undermined public security.

Sunday’s cancelled rally turn-out was expected to be larger than other protests since the elections because of anger over military police opening fire on garment workers demanding a doubling of the minimum wage.

The local human rights group LICADHO described Friday’s clash as the “worst state violence against civilians in Cambodia in 15 years”.

The United Nations special rapporteur Surya Subedi has called for an investigation into whether excessive force was used.

Lang Rith, a 29-year-old demonstrator from southern Takeo province, said he was hit with a baton on his back as he tried to run away on Saturday from Freedom Park which was a base for the protest movement since December 15.

“They beat us like they beat animals. I am very scared,” Lang Rith told Associated Press.

Protests have grown in recent weeks as garment workers joined opposition supporters demanding a re-run of the election it claims was rigged to allow Mr Hun Sen to remain in power.

An estimated 600,000 workers in 800 garment and footwear factories have been on strike since December 24 over a demand to raise the minimum wage to $US160 ($180) a month - $US60 higher than the government’s latest offer.

The strike has forced many of the factories to halt production.

Garments and footwear, including popular brands like Gap, Nike and H&M, are Cambodia’s largest exports and a key source of foreign earnings in the country, which remains one of Asia's poorest.

The decision to force protesters from Freedom Park came after the collapse of planned talks between the government and opposition leaders.